This invention relates to a spectral source and particularly to a spectral source for atomic absorption spectrometry.
In "Spectrochimica Acta" Vol. 28B, pages 51 to 63, a spectral source is disclosed which comprises a hollow cathode lamp including a hollow cathode and a rod-shaped anode, and a microwave power source disposed outside the hollow cathode lamp. D.c. current flowing between the anode and the cathode causes sputtering of the cathode, thereby producing atomic vapors. The atomic vapors are excited by the microwave power so as to emit light of a desired spectrum.
One essential disadvantage of this arrangement resides in the fact that due to the application of microwave power to the atomic vapors, atoms of some of the metals of which the hollow cathode consists will adhere to the interior wall of the lamp bulb, thereby considerably decreasing the lamp life time and the radiation intensity. Increasing the microwave power and thereby the radiation intensity will also increase noise and drift rates, thereby again shortening the life time of the lamp.
The specification of U.S. Pat. No. 3,893,768 to Stephens discloses a Zeeman modulated spectral source which comprises a lamp having an anode and a cathode formed by parallel flat plates of identical shape disposed opposite to each other, and means for applying a magnetic field to the space formed between the anode and the cathode. A d.c. current is caused to flow between the anode and the cathode to cause both cathode sputtering and excitation of the atomic cloud to produce the emission of light having the desired atomic spectrum, the light being Zeeman modulated by the magnetic field. Stephens indicates that a high-frequency or microwave generator may be used as a potential source instead of the d.c. source.
It is the essential disadvantage of this apparatus that the cathode sputtering cannot be performed sufficiently, even by using a high-frequency generator as the power source, because the ions produced by the discharge are caught in the magnetic field due to the identical flat shape of the opposite electrodes so that insufficient collisions between the ions and the cathode take place. Such insufficient cathode sputtering renders the radiation caused by excitation insufficient so that the intensity of the light emitted by the lamp will be low. By raising the high-frequency power in order to increase the cathode sputtering, the wear of the electrode will be increased thereby reducing the life of the lamp. While it is possible to achieve the cathode sputtering by the discharge the atomic vapors produced by sputtering are diffused due to the flat and uniform shape of the parallel electrodes.
Moreover, by applying the magnetic field to the lamp, the voltage for starting the discharge is raised. When the magnetic field is increased to intensify the light emission, the voltage for starting the discharge is raised even more. Although the cathode sputtering is thereby enhanced in quantity, the rise of the voltage for starting the discharge will increase the ion acceleration for cathode sputtering thereby considerably augmenting the wear of the electrode.
It is therefore an object of this invention to provide a spectral source having a long life time.
Another object of the invention is to provide a spectral source which emits radiation of high intensity.
Another object of the invention is to provide a spectral source in which the dimensions of the lamp can be reduced.
Another object of the invention is to provide a spectral source which can achieve a sufficient cathode sputtering so as to create a strong excitation radiation.
An important feature of the invention is the provision of a spectral source which comprises a lamp having a bulb, a first electrode disposed within the bulb and containing an element emitting a desired spectrum, a second electrode having a shape different from that of the first electrode, a high-frequency source connected between said first and second electrodes for establishing a high-frequency discharge therebetween to cause sputtering of the first electrode and excitation of a radiation with said desired spectrum, a gas contained in the bulb for maintaining said discharge, and a window provided by the bulb for transmitting said radiation.
It has been found that by providing two electrodes having different shapes and by applying high-frequency power to the space between those electrodes, a sufficient cathode sputtering, an intensive excitation radiation and a long life of the lamp may be obtained. Simultaneously, the dimension of the bulb, especially the length of the bulb along the direction of light emission, may be reduced.
The above and further objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.